Transistors, such as MOSFETs or IGBTs etc., are widely used for controlling or switching loads in different kinds of applications, such as in inverters, voltage regulators, current regulators, or drive circuits for driving electric loads, such as lamps, valves, motors, etc. Such transistors usually are n- or p-channel power transistors having a plurality of identical transistors cells arranged in a transistor cell field and electrically connected in parallel.
During operation, an electric current through a load path of the transistor is switched on or off or adjusted to a certain value by applying a control voltage to a contact terminal. The load path is formed between a source or emitter electrode on the one hand and a drain or collector electrode on the other hand.
Depending on the particular application in which a power transistor is used, the load path is usually connected in series with a load to be controlled or with a further power transistor between a positive and a negative electric supply potential, e.g. as bridge configuration. In such arrangements, the power transistor is either employed as a high-side (HS) transistor or as a low-side (LS) transistor. In case of a HS transistor, its load path is electrically connected between the positive supply potential and the load or the further power transistor. Accordingly, in case of a LS transistor, its load path is electrically connected between the negative supply potential and the load or the further power transistor.
Currently available power transistors may be operated at high temperatures, for instance at more than 150° C., at more than 175° C., or even at more than 200° C. However, if a power transistor's temperature exceeds a maximum permissible temperature, the transistor can be damaged or destroyed. Hence, in order to detect the transistor's temperature, many modern power transistors like those described in U.S. Pat. No. 6,876,043 B1 or in U.S. Pat. No. 7,835,129 B2 have an integrated temperature sensor element formed by the drain-body-diode, wherein the cathode of the drain-body-diode is connected to drain. In case of an n-channel transistor the use in HS configuration is unproblematic. In LS configuration however, there is always a heating-up if the load path of the power transistor is switched on and an electric current passes through it. In that on-state, the difference between the drain- and source-potentials of the power transistor amounts to only some 100 mV which is not sufficient for reasonably evaluating the signal. Hence, in such a configuration a reasonable temperature measurement is only available if the power transistor is switched off, that is, in a state in which the transistor cools down.
Therefore, there is a need for a solution that allows for detecting the temperature of a power transistor even in state in which the power transistor is switched on.